How to install applications on Ubuntu, through internet.

Before do this you have to set the repositories properly. Go to the

• System -> Administration -> Software Sources
  

Then do as following images. (if you want you can add more third party repositories. Like in second image.)

Then it will prompt a message. Then click on 'Reload' button. After this you can install application though terminal or Synaptic package manager (System -> Administration -> Synaptic Package Manager).
use the following command to install through the terminal.
sudo apt-get install "name of the application"
For an example if you want to install VLC Player then,
sudo apt-get install vlc

In Synaptic Package Manager you can simply search the application that you want and mark them for installation and then click on apply.